1. Field of the Invention
This invention is directed to solid electrolytes containing a solvent and, in particular, a solvent comprising a substituted cyclic ether, and optionally, an organic carbonate. This invention is further directed to solid electrolytic cells (batteries) containing an anode, a cathode and a solid electrolyte containing a solvent comprising a substituted cyclic ether. This invention is also directed to methods for enhancing the cycling efficiency of the solid electrolytic cells by employing a solid electrolyte which contains a solvent comprising a substituted cyclic ether.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features. Notwithstanding their advantages, the use of these solid batteries over repeated charge/discharge cycles is substantially impaired because these batteries typically exhibit significant drops in their charge and discharge capacity over repeated cycles as compared to their initial charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte inerposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable inorganic salt. The inorganic matrix may be nonpolymeric [e.g., .beta.-alumina, silicon dioxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic constituents include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes may also contain a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
The solid, solvent-containing electrolyte is typically formed in one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic salt and the electrolyte solvent are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
Notwithstanding the above, the initial capacity of solid batteries is often less than desirable. Moreover, even when the initial capacity of the solid battery is relatively high, such solid batteries often exhibit rapid decline in capacity over their cycle life. The lithium anode in such a cell may undergo a slow chemical change thereby limiting the cell's cycle life.
Specifically, the cumulative capacity of a solid battery is the summation of the capacity of a solid battery over each cycle (charge and discharge) in a specified cycle life. Solid batteries having a high initial capacity but which rapidly lose capacity over the cycle life will have low cumulative capacity which limits the effectiveness of these batteries for repeated use.
Secondary batteries can be recharged because they incorporate highly reversible chemical-electrochemical reactions to generate electrical energy. However, there are limits to how many times a battery can be recharged; and when the battery no longer has a reasonable recharge capacity (amp hours) or becomes unchargeable, it is considered to have failed. Reasons for such failures include corrosion, loss of active material on the electrode by mechanical failure, electrolyte erosion of active material, dissolution and nonredeposition of active material, and formation of inactive or nonconducting species.
A particular difficulty with electrolytic solvents heretofore used in solid solvent-containing electrolytes is their volatility and low flash points. Examples of such known solvents include dioxolane and tetrahydrofuran (THF). THF, for example, boils at 65.degree. C. and has a much lower flash point. Safety considerations require a search for lower volatility solvents which maintain or surpass the favorable performance of known solvents for solid solvent-containing electrolytes such as dioxolane.
In view of the above, the art is searching for methods to safely enhance the cumulative capacity of such solid batteries. It goes without saying that increases in the cumulative capacity of solid batteries would greatly facilitate their widespread commercial use.